Muscle Architecture

Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle’s mechanical function. There are several different muscle architecture types including: parallel, pinnate and hydrostats. Force production and gearing vary depending on the different geometries of the muscle. Some parameters used in architectural analysis are muscle length (Lm), fiber length (Lf), pennation angle (θ), and physiological cross-sectional area (PCSA)

Architecture types

Parallel and pennate (also known as pinnate) are two main types of muscle architecture. A third subcategory, muscular hydrostats, can also be considered. Architecture type is determined by the direction in which the muscle fibers are oriented relative to the force-generating axis. The force produced by a given muscle is proportional to the cross-sectional area, or the number of parallel sarcomeres present.

Parallel

The parallel muscle architecture is found in muscles where the fibers are parallel to the force-generating axis.These muscles are often used for fast or extensive movements and can be measured by the anatomical cross-sectional area (CSA). Parallel muscles can be further defined into three main categories: strap, fusiform, or fan-shaped.

Strap

Strap muscles are shaped like a strap or belt and have fibers that run longitudinally to the contraction direction.[4] These muscles have broad attachments compared to other muscle types and can shorten to about 40%-60% of their resting length. Strap muscles, such as the laryngeal muscles, have been thought to control the fundamental frequency used in speech production, as well as singing. Another example of this muscle is the longest muscle in the human body, the sartorius.

Fusiform

Fusiform muscles are wider and cylindrically shaped in the center and taper off at the ends. This overall shape of fusiform muscles is often referred to as a spindle. The line of action in this muscle type runs in a straight line between the attachment points which are often tendons. Due to the shape, the force produced by fusiform muscles is concentrated into a small area. An example of this architecture type is the biceps brachii in humans.

Fan-shaped

The fibers in fan-shaped muscles converge at one end (typically at a tendon) and spread over a broad area at the other end. Because of this, some consider muscles with this relative shape to be in a separate architecture type known as convergent muscle. Fan-shaped muscles, such as the pectoralis major in humans, have a weaker pull on the attachment site compared to other parallel fibers due to their broad nature. These muscles are considered versatile because of their ability to change the direction of pull depending on how the fibers are contracting.

Typically, fan-shaped muscles experience varying degrees of fiber strain. This is largely due to the different lengths and varying insertion points of the muscle fibers. Studies on ratfish have looked at the strain on fan-shaped muscles that have a twisted tendon. It has been found that strain becomes uniform over the face of a fan-shaped muscle with the presence of a twisted tendon.

Pennate

Unlike in parallel muscles, pennate fibers are at an angle to the force-generating axis (pennation angle) and usually insert into a central tendon.Because of this structure, fewer sarcomeres can be found in series, resulting in a shorter fiber length. This further allows for more fibers to be present in a given muscle; however, a trade-off exists between the number of fibers present and force transmission. The force produced by pennate muscles is greater than the force produced by parallel muscles.Since pennate fibers insert at an angle, the anatomical cross-sectional area cannot be used as in parallel fibered muscles. Instead, the physiological cross-sectional area (PCSA) must be used for pennate muscles. Pennate muscles can be further divided into uni-, bi- or multipennate.